1. Field of the Invention
The present invention relates to a Hall element for detecting a magnetic field. More particularly, the present invention relates to a planar Hall element having electrodes formed on a surface of the device and being capable of detecting a magnetic field in a direction perpendicular to that surface.
2. Description of the Prior Art
A Hall element can be used as a magnetic field sensor. Since current causes a magnetic field, a Hall element can also be used as a current detector, which, for example, might be applied in an electricity counter.
FIG. 1 is a view of a Hall element. In the Hall element, current supply electrodes C1 and C2 are positioned on opposite sides of a semiconductor substrate 1. Sensor electrodes S1 and S2 are positioned on the other opposite sides. A magnetic field is applied in a direction perpendicular to FIG. 1 (i.e., into the paper on which FIG. 1 is printed, as is represented by arrow B).
When a current flows between current supply electrodes C1 and C2 with a magnetic flux, an electromotive force, i.e., a Hall voltage, occurs between sensor electrodes S1 and S2. Assuming that the current has a current density J, and the magnetic flux has a magnetic flux density B, and further assuming that an x-axis exists in the direction from current supply electrode C1 to current supply electrode C2, a y-axis exists in the direction from sensor electrode S1 to sensor electrode S2, E.sub.y =R.sub.H .times.J.sub.x .times.B.sub.z will be satisfied, where E.sub.y is a y-axis component of an electric field, J.sub.x is an x-axis component of current density J, B.sub.z is a z-axis component of magnetic flux density B, and R.sub.H is a Hall coefficient.
In the Hall element of FIG. 1, L represents the distance between current supply electrodes C1 and C2, and W represents the distance between sensor electrodes S1 and S2. The rate of L/W must be large to obtain a large Hall voltage.
U.S. Pat. Nos. 4,782,375 and 4,987,467 disclose Hall elements whose electrodes C1, C2, S1 and S2 are situated on a surface of a semiconductor substrate 1, i.e., a planar type semiconductor device. FIG. 2 shows a typical Hall element of the type disclosed in the aforementioned patents. In FIG. 2, current supply electrode C1 is positioned at a central location between current supply electrodes C2. Sensor electrodes S1 and S2 are positioned between current supply electrodes C1 and C2. Since a current I between current supply electrodes C1 and C2 has a current component perpendicular to substrate 1, the Hall element of FIG. 2 detects a magnetic field parallel to the surface of substrate 1.
The dimensions L and W from FIG. 1 are also shown in FIG. 2. In FIG. 2, W substantially corresponds to the width of current supply electrode C1, and L substantially corresponds to the distance between current supply electrodes C1 and C2. The Hall element shown in FIG. 2 requires a large current component perpendicular to substrate 1. Therefore, the width of current supply electrode C1 must be large which, in turn, increases the size of W. This means that L must also be large to ensure that L/W will remain satisfactorily large. The overall result is that the size of the Hall element shown in FIG. 2 is large.
An offset voltage, which would exist without any magnetic field, is another problem. An offset voltage is sometimes caused by the asymmetrical placement of electrodes C2, S1, S2 with respect to electrode C1. Also, an offset voltage is sometimes caused by stress, since the mobilities of carriers may change due to the piezo effect and, also, the amount of piezo effect is different depending upon the crystal direction. For example, piezoresistances change at a 90.degree. period in the &lt;100&gt; plane of silicon (Si). The current shown in FIG. 2 has a component flowing close to the surface of substrate 1. Since stress is large at that surface, the offset voltage problem still remains.
A Hall element may be positioned within a gap in a yoke wound by a wire carrying an object current to detect a magnetic field. This gap must be narrow, because a magnetic field applied to a Hall element is in inverse proportion to the distance of the gap. However, since the magnetic field direction in the Hall element shown in FIG. 2 is parallel with respect to the substrate, the gap will be unavoidably large in the case where such a Hall element is inserted in the gap.
As can be understood from the above, the sensitivity of a current detector using a planar type Hall element is not sufficient.